Definitely DO NOT use all-thread for the lead screws!! Using 1/2-10 single start will run 2x the speed of 1/4-20 and have less tendency to whip. These will make a very usable machine but you will eventually think that it is also too slow. www.use-enco.com sells these 6' utility grade rods for around $9 or less, which is less than 1/2" x 10' all-thread in some cases.
Using 1/2-10 5 start ACME is highly recommended for the higher torque steppers like 425 oz/in and will run even faster with 1/2-10 8 start. These come at a higher price but they are well worth it. McMaster-Carr has these and the last time I looked the 8 start was a couple dollars cheaper than the 5 start. These are precision rods. Use DumpsterCNC anti-backlash lead nuts with these lead screws.